All current antibiotics, with the exception of isoniazid (INH) and ethionamide, inhibit bacterial growth by inhibiting bacterial RNA, DNA, protein or cell wall synthesis. While bacteria and mammalian cells synthesize lipids via pathways that are similar in principle, the enzymes that catalyze bacterial lipid synthesis differ in fundamental respects from their mammalian counterparts. Bacteria, especially M. tuberculosis (M.tb.), contain unique lipids not found in mammalian cells. We have discovered that the lypolipidemic drug gemfibrozil (GFZ), which has been used safely in humans for >20 years, blocks growth of 27 different pan-drug sensitive and multi-drug resistant strains of M tb. as well as 10 other species of bacteria. GFZ exerts a bactericidal effect on L. pneumophila, both in bacterial growth medium and in macrophages. Thus metabolites found in mammalian cells do not block GFZ's inhibitory effect on L. pneumophila. We have screened >10(12) L. pneumophila colonies but have found no GFZ-resistant mutants. This suggests that GFZ acts on highly conserved, hard to mutate enzyme(s). The 3- and 6-propylene analog of GFZ are 5- fold more potent than GFZ in blocking 14C-acetate incorporation into L. pneumophila lipids. Other fibric acids such as clofibrate and bezafibrate, are ineffective. We have identified an L. pneumophila enoyl reductase (Lpn FabX) that is GFZ's presumptive target, purified the enzyme and showed it is competitively inhibited by GFZ's CoA adduct (GFZ-CoA), but not by GFZ. GFZ-CoA also competitively inhibits InhA, the M.tb. enoyl reductase that is a target of INH. L. pneumophila converts 3H-GFZ to 3H- GFZ-CoA in vivo. GFZ-CoA is the first competitive inhibitor of a bacterial enoyl reductase to be identified. Funds are requested to explore the mechanisms by which GFZ inhibits M.tb growth, and to test the effects of GFZ and of its 3- and 6-propylene analogs, alone and in combination with other anti-tuberculosis drugs, on M.tb growth in macrophages and in mice.